Susan Sontag has returned photography to the cockpit of discussion it occupied when the exact mechanical image loomed as a threat to the person, to art, to the very relationship between images and reality. The last, essentially, is Sontag's subject, approached--after a splatter of (as yet) unsupported assertions --via touchstone figures: writers, photographers, painters interchangeably. (The book has no illustrations; it assumes, reasonably enough, a common stock of photographic images.) In a vivid, close-set argument, she traces Whitman's theme, "the levelling of distinctions between the beautiful and the ugly, the important and the trivial," through Lewis Hine and Walker Evans to its "last sigh," the 1955 Family of Man exhibit, apex of "sentimental humanism"--and jumps to the toast of 1972, Diane Arbus, in whose world "everybody is an allen." But levelling down, Arbus-like, is also "lowering the threshhold of what is terrible," as much modern art does, as Surrealism does systematically: "all subjects are merely objets trouvÃ‰s." So we are confronted with photography, reputedly realistic, as the art "that has best shown how to juxtapose the sewing machine and the umbrella," and with the photographer as "the voyeuristic stroller who discovers the city as a landscape of voluptuous extremes." Beauty falls, morality falls, as a standard; "photographic seeing" is the criterion, following "the idea that everything in the world could be made interesting through the camera." So photographs become--Sontag adduces a misconstruction of Proust's--"not so much an instrument of memory as an invention or replacement." Images, that is. Dismemberments of reality. The Chinese want only complete, correct views, Sontag observes in a stunning windup. For them, a "good" picture; for us, a good picture. With an anthology of quotations (also shards of reality) from the unlikes of Daguerre, Man Ray, and a 1976 Minolta ad for further agitation.